The present invention relates to the diagnostic imaging arts. It finds particular application in conjunction with diagnostic imaging with MRI scanners for magnetic resonance angiography (MPA) and will be described with particular reference thereto. It will be appreciated, however, that the invention is also applicable to other types of magnetic resonance imaging and spectroscopy.
Magnetic resonance angiography is used to view the blood vessels of the body. Dipoles in the blood of the subject are excited and imaged as they propagate through vessels of interest. A clinician identifies various circulatory abnormalities, such as slow points or partial blockages within the vessel with the image.
Typically, a blood vessel is imaged with its flow perpendicular to the imaging slices of a slab or volume of interest. The resonance signal of the blood tends to degrade as it passes to deeper slices within the imaging region. In a typical scan of stationary tissue, dipoles are subjected to an initial excitation pulse, then to a series of refocusing pulses, as the sequence dictates. The refocusing pulses help position the dipoles such that the next excitation pulse will have greatest effect. In magnetic resonance angiography, the dipoles being imaged are in motion. A typical phenomenon is that dipoles pass from one imaging slice into another before being refocused, and into yet another slice before being subjected to subsequent excitation pulses. This results in partial saturation of the magnetic resonance signal, and manifests in signal degradation as imaging goes on. In an oblique image along the vessel, the spatially varying diameter vessel appears to taper in signal intensity across the imaging volume. Obviously this has ramifications in diagnosis, as it becomes difficult to tell the difference between a constricted vessel and an imaging artifact.
Previously, ramped RF pulses, e.g. TONE, RAMP, or VUSE RF pulses as they are known in the art, have been used to counteract such phenomena. That is, the RF pulse is designed to affect each successive slice simultaneously with a greater tip angle. As blood passes from slice to slice, the RF pulses affect the blood in such a manner to minimize signal saturation.
The slope of the ramp is determined empirically based on a simple model and assumptions, not tailored optimally or specifically to each individual subject. These pulse profiles are designed from prior tests, but are not always sufficient for eliminating saturation artifacts. Many factors ultimately affect how the blood behaves as it traverses the imaging region. These factors include, but are not limited to, patient height, weight, sex, age, profession, and blood pressure. In short, the behavior varies from patient to patient, region to region within the same patient, and can even vary from imaging session to imaging session of the same region of the same patient.
The clinician selects a ramped RF pulse sequence based on personal experience. The volume scan is conducted, typically about 8-15 minutes. If the image has artifacts that indicate that a less than optimal ramp slope was selected, the process is repeated or the diagnosis is made from flawed images.
The present invention provides a new and improved method and apparatus which overcomes the above-referenced problems and others.
In accordance with one aspect of the present invention, a magnetic imaging method is provided. Magnetic resonance is excited in a plurality of subregions, containing some measure of flowing material with radio frequency excitations having spatially adjustable tip angles. Magnetic resonance signals from the sub-regions are measured. The tip angle effected in each sub-region is adjusted according to the measured resonance. A volume image representation is generated using the adjusted tip angles.
In accordance with another aspect of the present invention, a method of magnetic resonance angiography is given. Magnetic resonance is excited in and received from a volume within a subject. A radio frequency pulse sequence is designed based on the received resonance, and used in a scan.
In accordance with another aspect of the present invention, a magnetic resonance apparatus is provided. A main magnet assembly generates a main magnetic field through a subject in an imaging region. A gradient assembly spatially encodes the main magnetic field, and a radio frequency assembly excites and manipulates magnetic dipoles within the subject. An intensity analyzer receives at least one data line from subregions of the imaging region, and determines a signal intensity for each imaging region. A radio frequency pulse sequence synthesizer adjusts tip angles of RF pulses for each subregion in accordance with measured intensities.
According to another aspect of the present invention, a magnetic resonance apparatus is provided. It includes a means for determining flow parameters of a fluid in a vessel of interest, such as flow rate, velocity, shear rate, and the like. It has a means for adjusting a radio frequency pulse profile based on the flow rate. It has a means for applying the adjusted profile and spatially encoding the fluid. It has a means for demodulating resonance signals and reconstructing them into image representations.
One advantage of the present invention resides in its improved visualization of blood vessels.
Another advantage of the present invention is reduced scan time, improving image quality vs. Scanning time efficiency.
Another advantage of the present invention is reduction of the slab-boundary artifact.
Another advantage resides in the dynamic, real time fitting of scan parameters to each patient.
Still further benefits and advantages of the present invention will become apparent to those skilled in the art upon a reading and understanding of the preferred embodiments.